1. Field of the Invention
This invention is related to improvements in dynamic information storage or retrieval devices, such as mass data storage devices, or the like, and to improvements in components thereof and methods of operating same, and more particularly to improvements in methods and apparatuses for establishing coefficients used in an FIR filter for use in such dynamic information storage or retrieval devices, or the like to enable a fast EPR4 equalization start up.
2. Relevant Background
In the construction of dynamic information storage or retrieval devices, or the like, and in particular in the construction of the data channel used in digital magnetic recording systems, such as hard disk drives used in conjunction with digital computer systems, or the like, there has been significant recent interest in Partial Response Maximum-Likelihood (PRML) signaling techniques. The most common PRML systems are PR4ML ( a partial response class 4). Maximum-likelihood detectors, which use a Viterbi algorithm, are generally used for these partial response channels. In such systems, maximum-likelihood detection techniques are widely used, and involve probabilistic techniques for determining data states in the data channel.
The PRML equalization of a magnetic recording read channel is usually accomplished by a continuous-time filter, CTF, that is implemented using analog techniques, followed by a finite-impulse response filter, FIR. The FIR filter can be implemented in one of two ways: either in a sampled analog fashion or in a pure digital fashion. In the former case, the CTF and FIR a sample-and-hold circuit separates the circuits, whereas in the latter case, an analog-to-digital converter, ADC, separates the two blocks. The proposed method is applicable to both kinds of FIR implementations. A special xe2x80x9cindirect EPR4 modexe2x80x9d additionally includes a 1+D operation block that transforms PR4 equalized samples into EPR4 equalized samples.
The chief purpose of the CTF filter is to provide an anti-aliasing filter to limit the spectral contents of the signal and noise beyond the Nyquist frequency. However, the CIF filters are inherently difficult to tune and adapt. It is the FIR filter, nowadays, that plays the major role in achieving the fine signal equalization to the desired PRML target. With the advent of deep submicron CMOS processes, the digital FIR filters with sophisticated Least Mean Square, LMS, coefficient adaptation algorithms are becoming less and lest costly, whereas the analog CTF circuits do not scale down very well.
Setting the CTF filter parameters, such as boost, bandwidth and zero asymmetry and finding a good initial value of the FIR coefficients for an LMS adaptation of a EPR4 read channel system is quite a difficult and time-consuming task. If the FIR initial coefficients are not set correctly, the LMS adaptation will not converge and another trial is needed. Further, for a direct EPR4 system, the equalization, timing and gain loops are less robust and more interdependent. A trial of many initial guess settings dramatically increases the testing time.
Moreover, the LMS adaptation of FIR coefficients is customarily done using a sign-sign LMS algorithm. Here, only a sign bit of the unequalized input data sample and the corresponding equalized output data sample error are used instead of their full value representations. The three-level PR4-target adaptation is quite robust and normally guarantees the adaptation convergence. The five-level EPR4 -target adaptation, on the other hand, because of the narrower xe2x80x9ceye-pattern openingxe2x80x9d, is less robust and less likely to converge if the wrong initial setting of coefficients is used.
The proposed method utilized the robustness of the three-level PR4-target LMS adaptation of the PR4/EPR4 read channel in order to establish an initial setting of the coefficients of a newly-built system, that is, a system with not very well known channel characteristics. With the thus obtained FIR coefficients being in the neighborhood of the final solution, a more refined but less robust (in face of an incorrect initial setting), adaptation method can be used.
The method of the invention uses a multistep approach, each step being of increasing accuracy, to achieve the ultimately fine equalization of the EPR4 read channel system may be used in conjunction with either indirect or direct EPR4 system operating modes. In an indirect equalization system, the first step is to obtain a rough estimate of the FIR coefficients by performing a robust three-level adaptation at the FIR output. The second step is to switch the source of the FIR equalized data sample error from the three-level FIR output to a five-level output, using a 1+D operation to obtain the five-level output. The LMS adaptation performed on the five-level detector input will result in a better system performance with lower bit error rate.
For the direct EPR4 system operating mode in which the FIR output is directly equalized to the five-level EPR4 targets and the 1-D operation is not ultimately needed, the same steps are performed, and, additionally, a third step is performed to bypass the 1+D operation so that the FIR output data feeds directly to the detector. The FIR output is also directly fed to the LMS block to derive the equalized data samples error. The final FIR coefficients are changed in value from those of the second step of the indirect operating mode, since the 1+D operation is absorbed by the FIR.
Thus, according to a broad aspect of the invention, a method is presented for establishing filter coefficients of an FIR filter in a read channel of a mass data storage device. The method includes using a PR4 adapted signal from the FIR filter in an LMS adaptation circuit to adapt the coefficients of the FIR filter, and after a predetermined time, using an EPR4 adapted signal from the FIR filter to adapt the coefficients of the FIR filter.
According to another broad aspect of the invention, a method is presented for achieving a fast EPR4 equalization start-up for an FIR filter having an output that is directly equalized to five-level EPR4 targets in a magnetic recording read channel. The method uses a PR4 adapted signal form the FIR filter in an LMS adaptation circuit to adapt the coefficients of the FIR filter, and after a predetermined time, using an EPR4 adapted signal from the FIR filter to adapt the coefficients of the FIR filter. Thereafter, the EPR4 signal is used directly from the FIR filter. A 1+D operation may be applied to the PR4 signal from said FIR filter to produce the indirect EPR4 target, which may be used for the initial LMS adaptation. Thereafter, the EPR4 signal directly from the FIR filter may be used by bypassing the 1+D operation.
According to yet another broad aspect of the invention, a method for achieving a fast EPR4 equalization start-up for an FIR filter having an output that is directly equalized to five-level EPR4 targets in a magnetic recording read channel includes using a PR4 adapted signal from the FIR filter in an LMS adaptation circuit to adapt the coefficients of the FIR filter, after a predetermined time, using an EPR4 adapted signal from the FIR filter to adapt the coefficients of the FIR filter. LMS coefficients from the EPR4 signal from the FIR filter may then be recalculated, for example, by an associated controller to obtain a final EPR4 target.
According to still yet another broad aspect of the invention, an apparatus is presented for achieving a fast start-up EPR4 equalization for an FIR filter having an output that is equalized to three-level PR4 targets in a magnetic recording read channel of a mass data storage device. The apparatus includes a 1+D circuit connected to receive said FIR filter output and to provide an EPR4 target output, and a first multiplexer to receive and selectively output said FIR filter output and said EPR4 target output. An LMS adaptation circuit is connected to receive said output of said multiplexer to generate therefrom modified FIR filter coefficients for use in said FIR filter. The three-level PR4 target signal can be selected for first application to said LMS equalization circuit for gross coefficeint adaptation and said five-level EPR4 target signal can be selected for second application to said LMS equalization circuit for fine coefficeint adaptation.
According to yet another broad aspect of the invention, an apparatus is presented for achieving a fast start-up EPR4 equalization for an FIR filter having an output that is equalized to five-level EPR4 targets in a magnetic recording read channel of a mass data storage device. The apparatus includes a circuit connected to receive the FIR filter output and to provide an indirect EPR4 target output to a first multiplexer. The multiplexer receives and selectively outputs either the FIR filter output or an indirect EPR4 target output. An LMS equalization circuit is connected to receive the output of the multiplexer to generate therefrom modified FIR filter coefficients for use in the FIR filter. Thus, the multiplexer can be initially configured to select PR4 equalized target coefficients from the FIR filter for first application to the LMS equalization circuit for gross coefficient adaptation, to select an indirect five-level EPR4 equalized target signal for second application to the LMS equalization circuit for finer coefficient adaptation, and to select a final direct EPR4 output from the FIR for output to a detector circuit.
In light of the above, therefore, one principal advantage of the invention, is that a robust set-up technique for an EPR4 -target can be provided that will easily converge, without the concerns previously encountered in which difficulty in convergence may be encountered using five-level EPR4 -target adaptation.
These and other objects, features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of the invention, when read in conjunction with the accompanying drawings and appended claims.